Building an automatic waterer. What keeps the water clean?

[Turtle5600]

I'm going to start building an automatic rain collection waterer with the cup type drinkers.I also intend on using an inline filter to keep any accidental trash out of the line going to the drinking cups. I'm wondering what I can use to keep the water from turning green. The little 3.5 gallon waterer I have now turns green quickly and cleaning it out and filling it up every day is getting annoying. Would a small amount of bleach every so often be ok or is there some sort of water purifying tablet that I can use. I'm thinking of wither using a plastic 275g tote or 55g drum, leaning the route of the tote right now. I have 13 birds and would like to be completely sufficient off of rain water from the top of the coop.